Lubricants are often used in demanding environments where degradation can be accelerated by high temperatures, extreme wear conditions, acidic or other corrosive conditions, etc. For example, the conditions under which automobile engines function are severe enough to require periodic oil changes to replace degraded engine lubricant in order to protect the engine against wear and damage that can lead to catastrophic failure.
Alkylated diaryl amines, such as alkylated diphenylamines (ADPAs), are well known antioxidants widely used to prevent degradation and maintain the performance of engine oils found in gasoline and diesel engines for cars and trucks, as well as a variety of industrial lubricants and lubricants for marine engines, etc. When selecting a diaryl amine antioxidant a number of performance, safety and environmental concerns must be addressed. For example, diphenylamine itself has good antioxidant activity but is known to be a sensitizer and its presence is typically kept to a minimum, e.g., less than 1% of any ADPA antioxidant. Diphenylamines substituted with hydrocarbyl groups are more soluble in lubricating oil and the higher molecular weight reduces volatility. Increased alkylation also helps to solubilize polar materials formed from oligomerization of spent oxidized amines, which reduces deposits, sludge and varnish. On the other hand, the antioxidant activity of ADPAs is dependent on the concentration of nitrogen provided and is thus inversely proportional to molecular weight and so excessive alkylation or very large alkyl groups should be avoided. NAUGALUBE 438L, a mixture of diphenylamines alkylated by one or more nonyl-chains derived from propylene trimer is an effective and widely used liquid antioxidant.
Diaryl amines useful as anti-oxidants bearing substituents other than alkyl groups are known but such compounds are not as common in engine oils as alkyl substituted diaryl amines. For example, U.S. Pat. No. 7,704,931 includes 3-hydroxydiphenylamine and 4-hydroxydipenylamine in lists of possible antioxidants in a lubricant composition; U.S. Pat. No. 8,202,829 includes 3-hydroxydiphenylamine in a list of suitable antioxidants for use in a non-synthetic lubricating oil comprising less than 30 wt % monocycloparaffins and from 0.8 to 2.0 wt % tetracycloparaffins; and U.S. Pat. No. 7,569,526 includes 3-hydroxydiphenylamine and 4-hydroxydipenylamine in lists of possible antioxidants for use in the oil portion of a metal working fluid, but none of these three disclosures exemplify the use of a hydroxydiphenylamine.
U.S. Pat. No. 7,498,467 disclose aminophenol and hydroxydiphenylamine antioxidants wherein on at least one phenyl ring a hydroxyl substituent is adjacent to an amino substituent.
JP 2011-256314 discloses a composition comprising an aliphatic alkyl ester biodiesel fuel, which fuel may also contain a fossil fuel component, and an antioxidant of the formula
wherein n and m can be 0, 1 or 2 provided that m+n=1 or 2, and R is a C1-18 alkyl, which alkyl may be further substituted. Of the possible hydroxydiphenyl amine compounds of the above formula, only 4-hydroxydiphenylamine is exemplified.
GB 1,145,189 discloses the use of substituted 2-hydroxydiphenylamines as antioxidants in hydrocarbon and ester based lubricating oils.
EP 016559 discloses 3-hydroxy-4-styryldiphenylamine, which may also be further substituted by styryl at the 2- or 4′-positions, as an antioxidant for hydrocarbon and ester based lubricating oils. Compositions comprising ester based oils are exemplified.
“Thermoanalytic study of inhibitors of oxidation of synthetic oils” Kyazim-zade, A. K.; Gadirov, A. A.; Akchurina, T. Kh., Neftekhimiya (1996), 36(1), 73-75 investigated the thermal stability and the effect on the oxidation of pentaerythritol esters at elevated temperatures of certain 3-hydroxyl or 3-alkoxy-4-hexyldiarylamines of the following formula:
wherein R is hydrogen, butyl or hexyl and R′ is hydrogen or methyl.
There is some suggestion in the literature that substitution with a hydroxyl group at the 2 and 4 positions of a diphenyl amine would lead to increased antioxidant activity, but hydroxyl substitution at the 3 positions is not expected to provide the same benefit, as discussed in, for example, “Substitution and torsional effects on the energetics of homolytic N—H bond cleavage in diphenylamines” Poliak, Peter; Vaganek, Adam; Lukes, Vladimir; Klein, Erik, Polymer Degradation and Stability (2015) 114, 34-44.
It has been found that certain alkylated 3-hydroxydiphenylamines, i.e., diphenylamines substituted on one phenyl ring by at least one hydroxyl at the 3-position relative to the amino group and substituted on the other phenyl by at least one group having 4 or more carbon atoms, provide excellent oxidation protection and deposit control for lubricants, such as lubricating oils used in automobile engines, truck engines, wind turbines, etc. In many embodiments, the alkylated 3-hydroxydiphenylamine antioxidants of the invention provide greater anti-oxidant and/or deposit control activity than hydroxydiphenylamines substituted by hydroxyl at the 2- and/or 4-position rather than the 3-position. Particularly good performance is seen when the alkylated 3-hydroxydiarylamines are used in combination with alkylated diarylamines, such as commercially available products like nonylated diphenylamine NAUGALUBE 438L.